This invention relates generally to gas turbine engines, and more specifically to turbine blades used with gas turbine engines.
At least some known gas turbine engines include a core engine having, in serial flow arrangement, a high pressure compressor which compresses airflow entering the engine, a combustor which burns a mixture of fuel and air, and a turbine which includes a plurality of rotor blades that extract rotational energy from airflow exiting the combustor the burned mixture. Because the turbine is subjected to high temperature airflow exiting the combustor, turbine components are cooled to reduce thermal stresses that may be induced by the high temperature airflow.
The rotating blades include hollow airfoils that are supplied cooling air through cooling circuits. The airfoils include a cooling cavity bounded by sidewalls that define the cooling cavity. Cooling of engine components, such as components of the high pressure turbine, is necessary due to thermal stress limitations of materials used in construction of such components. Typically, cooling air is extracted air from an outlet of the compressor and the cooling air is used to cool, for example, turbine airfoils. The cooling air, after cooling the turbine airfoils, re-enters the gas path downstream of the combustor.
At least some known turbine airfoils include cooling circuits which channel cooling air flows for cooling the airfoil. More particularly, internal cavities within the airfoil define flow paths for directing the cooling air. Such cavities may define, for example, a serpentine shaped path having multiple passes. Cooling air is directed through a root portion of the airfoil into the serpentine shaped path. Because thermal stresses may be induced into the internal cavities, walls defining the cavities may be coated with a environmental coating to facilitate preventing oxidation within the cooling cavity.
To facilitate withstanding internal thermal stresses, at least some known blades are coated with a layer of environmental coating that has a thickness approximately equal to 0.003 inches. Applying the environmental coating with such a thickness prevents oxidation of the cavity walls and facilitates the airfoil withstanding thermal and mechanical stresses that may be induced within the higher operating temperature areas of the blade. However, the presence of an environmental coating at such a thickness may cause a reduction in material properties in regions of the blade operating at a lower temperature, which may lead to cracking of the material. In time, continued operation may lead to cracking of the blade and/or a premature failure of the blade within the engine.
In one aspect of the invention, a blade for a gas turbine engine is provided. The blade includes a leading edge, a trailing edge, a first sidewall extending in radial span between a blade root and a blade tip, and a second sidewall connected to the first sidewall at the leading edge and at the trailing edge. The first and second sidewalls each include an outer surface and an inner surface. A cooling cavity is defined by the first sidewall inner surface and the second sidewall inner surface. At least a portion of the cooling cavity is coated with an oxidation resistant environmental coating that has a thickness less than 0.0015 inches.
In another aspect, a gas turbine engine including a plurality of blades including an airfoil is provided. Each airfoil includes a leading edge, a trailing edge, a wall, and a cooling cavity defined by the wall. The cooling cavity includes at least two chambers. A first of the chambers is bounded by the airfoil leading edge, and a second of the chambers is bounded by the airfoil trailing edge. A first portion of the cooling cavity is coated with an oxidation resistant environmental coating applied with a first thickness. A second portion of the cooling cavity is coated with an oxidation resistant environmental coating applied with a second thickness that is less than the first portion first thickness. More specifically, the second portion second thickness is less than 0.0015 inches.
In a further aspect, a method for manufacturing a blade for a gas turbine engine is provided. The method includes the steps of defining a cavity in the blade with a wall including a concave portion and a convex portion connected at a leading edge and at a trailing edge, and dividing the cavity into at least a leading edge chamber and a trailing edge chamber, such that the leading edge chamber is bordered by the blade leading edge, and the trailing edge chamber is bordered by the trailing edge. The method also includes the step of coating at least a portion of an inner surface of the wall with a layer of an oxidation resistant environmental coating having a thickness less than 0.0015 inches.